The diaphragm valve using a diaphragm and used in various types of chemical lines, pure water lines, etc. in the past, as shown in FIG. 6, is provided with a body 47 having an annular groove 51 formed thereon, a bonnet 48, and a diaphragm 49 having an annular engagement part 56. The annular engagement part 56 of the diaphragm 49 is engaged with the annular groove 51 of the body 47. The body 47 and the bonnet 48 clamp and fixedly hold the annular engagement part 56 and its peripheral edge 55 between them so as to seal between the body 47 and bonnet 48 and prevent leakage of fluid to outside the valve. However, with this seal method, long term fluid pressure fluctuations, temperature changes, etc. cause creep at the seal part configured in the above way. Therefore, there is a problem that the fluid can leak to the outside. This tendency is particularly remarkable in polytetrafluoroethylene (hereinafter referred to as “PTFE”), a tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (hereinafter referred to as the “PFA”), or other fluororesin preferably used as the diaphragm material.
In order to solve the above problem, the applicant previously invented and filed a patent application for a high safety diaphragm valve improved over the conventional diaphragm valve, extremely simple in structure, and free from fluid leakage to outside the valve even if creep occurs at the seal part (see Japanese Unexamined Patent Publication No. 6-2775). Explaining the structure with reference to FIG. 7, the diaphragm valve includes a body 57 having an annular groove 62 formed thereon, a bonnet 58, and a diaphragm 59 provided at the peripheral edge 66 thereof with an annular engagement part 67. The annular engagement part 67 of the diaphragm 59 is engaged with the annular groove 62 formed on the body 57. Further, the peripheral edge 66 of the diaphragm 59 is clamped between the body 57 and bonnet 58 via an elastic member 60 (O-ring).
However, in the above-mentioned diaphragm valve, a fluororesin is used for the material of the body, diaphragm, etc. Further, it is found that there is a danger of fluid leakage to the outside of the valve when the pressure or temperature of the fluid changes greatly over a long time or the valve is otherwise placed under conditions where the seal part is very susceptible to creep.